Preparation of alkali metal chloride melt for use in electrolysis of aluminum chloride

ABSTRACT

ADVANCE PREPARATION OF ALKALI METAL CHLORIDE MELT FOR USE IN ELECTROLYSIS OF ALUMINUM CHLORIDE.   D R A W I N G

Sept. 25, 1973 W E, HAUPlN ET AL 3,761,365

PREPARATION OF ALKALI METAL CHLORIDE MELT FOR USE IN ELEGTROLYSIS OFALUMINUM CHLORIDE Filed sept. 9, 1971 A Harney United States Patent OU.S. Cl. 204-67 26 Claims ABSTRACT OF THE DISCLOSURE Advance preparationof alkali metal chloride melt for use in electrolysis of aluminumchloride.

BACKGROUND OF THE INVENTION 'Field of the invention This inventionrelates to electrolytic production of aluminum from aluminum chloride.More particularly, it relates to preparation and purification of a meltof highpurity alkali metal chloride and introduction thereof t anelectrolytic cell for the production of aluminum from aluminum chloride.When we refer herein to the melt, We refer to the alkali metal chlorideelectrolyte before the aluminum chloride to be electrolyzed has beenadded to the cell. By alkali metal chloride, We mean any metal chlorideWhose decomposition potential is substantially above that of aluminumchloride, including alkaline earth metal chlorides such as magnesiumchloride and calcium chloride.

Description of the prior art The production of aluminum by theelectrolysis of aluminum chloride in an alkali metal chloride melt,although a theoretically feasible and long-sought objective 0f the art,has never been commercially realized because of numerous unsolvedtechnical problems which have precluded the economic continuousoperations requisite to commercial production. Among such problems is aprogressive deterioration in the operating efiiciency of theelectrolytic cell, with a marked change in its electrical operatingcharacteristics and diminution of its output of metallic aluminum.Although not fully understood at the present time, we believe thatcertain of the problems that have long been extant are attributable tothe presence of undesired constituents and finely divided particulatematerials in the molten salt bath. The particulate is attracted to thecathode, apparently by electrical forces, Where it forms a semipermeablecoating. This coating of oxides or other particulates on the cathodesurface operates to inhibit transport of the complex Al ion because ofits large size-to-charge ratio. In contrast, the alkali metal ions aredriven by the electrical potential gradient and, due both to theirabundance and a small size-to-charge ratio, easily penetrate theparticulate lm and are discharged at the cathode. These reduced species,particularly sodium and potassium, enter the graphite or like lattice,with consequent expansion thereof and surface soughing to further add tothe particulates present in the system. In this manner, the mobility ofaluminum chloride to the cathodic surface by both convection anddiffusion is markedly impeded.

Apart from the foregoing, the presence of oxide and hydroxide impuritiesin the melt also causes detrimental 3,761,365 Patented Sept. 25, 1973SUMMARY OF THE INVENTION We have found that the aforementioned problemsmay be minimized, if not effectively avoided, for extended periods oftime by forming a melt of at least one alkali metal chloride, theresultant melt containing at least about by weight alkali metalchloride, and feeding same to a cell in which AlCl is to be electrolyzedafter addition to the melt or electrolyte therein. According to ourinvention, the melt is preferably purified before being fed to the cell.Purification may be by any conventional means of separating solids, forexample, by settling, centrifuging, decanting, freezing and remelting,or the like. Our presently preferred method 0f preparing the melt is toform it in a separate heated chamber, as for example, a furnace, in anatmosphere of nitrogen, argon, or other inert gas which serves to purgethe chamber and prevent the introduction of moisture into the melt. Suchinert gas can also be used to force the melt to flow through a filterdisposed in the chamber according to a preferred embodiment of ourinvention, preparatory to its transfer to the electrolysis cell.

The one or more alkali metal or alkaline earth metal chlorideingredients of the melt may be added to the melting chamber in solidform, either mixed together or separately. If desired, some of the meltthat is tapped off with the aluminum removed from the electrolytic cellmay be separated from the aluminum and introduced to the pre-meltchamber for re-pun'fication prior to being conducted back to theelectrolytic cell for reuse.

Impurities or contaminants which may be present, for example, as aresult of being introduced during preparation of the alkali metalchloride melt, and not be in solid form, may be precipitated as solidsaccording to our invention. This enables them to be readily removed fromthe melt. For example, aluminum may be added to precipitate one or moremetal chlorides, as for example, in the case of iron of valence 3,present as FeCl3, in accord with the equation Al-l-FeClg-AlCl-l-Fe. Suchadded aluminum, in a slower reaction, also functions to remove someoxygen values if the same are present, as for example, as heavy metal(X) hydroxide, by the illustrative reaction 2nAl-l-3X(OH)nnAl2O3-{3X+72nH2, Where n is the Valence of the heavy metal. Wehave also found that addition of aluminum chloride to the pre-melt, inan amount not exceeding 1 to 2% by weight, further facilitates removalof soluble oxygen, as for example, by reaction to form a solublealuminum-oxygen-chlorine complex which reacts further to form insolubleA1203. However, if more than 1 to 2% by Weight of aluminum chloride isadded to the pre-melt, at least some of the so formed alumina is likelyto go back into solution, thereby at least partly defeating the purposeof such addition. The precipitated materials and any other insoluble orundissolved matter, including oxides, and perhaps carbon or the like,and precipitates and sludge formed therefrom, in the pre-melt willnormally settle to the bottom of the chamber in the form of a sludge,and thus be effectively separated from melt that is transferred into theelectrolytic cells. As mentioned hereinabove, any unsettled matterand/or particulate suspensions which may be present in the melt due tothermal fiow conditions such as convection currents r other factors, aswell as precipitated solids, may be removed by passage of the meltthrough a filter such as disclosed above and described hereinafter.

In effecting the removal of purified melt from the chamber it isdesirably displaced, preferably upwardly, through a conduit or the like,and may be conveniently filtered as it is introduced thereinto. Thefilter material should be alkali metal chloride-resistant and ofsufiicient permeability to selectively permit passage of the liquidalkali metal chlorides therethrough and to prevent the passage of muchof the undssolved or other solid particulate matter therethrough. Suchfilter may comprise, for example, material of fibrous character coveringor wrapped around a perforated metal, metal alloy (such as the nickelbase alloy Inconel), refractory or like plate or end section on a pipe,tube or like conduit which may extend into the aforementioned chamber ator to a point below the surface of the melt. The perforated plate or endsection serves as a support for the filter material. If desired, thefilter material may be secured thereto by means of one or more clamps orthe like. Such an arrangement permits easy flow of filtered melt intosuch conduit and on to an electrolysis cell to which proper quantitiesof AlCl3 may be added. Although we will describe herein a preferredembodiment of said filter, it should be understood that other filteringarrangement can be employed, such as a pad, or the like, of permeablemedium over a perforated annular entrance to a pipe or conduit whichextends into the chamber, or as a cover for wire screen or the like ofappropriate shape placed over an opening into a line which leaves thepre-melt chamber and leads to an electrolytic cell, regardless of wherethe opening may be positioned in the chamber, or in what direction theexiting melt may flow.

According to a preferred embodiment of our invention, a multi-elementfilter medium is employed which comprises at least one fibroussilica-containing or siliceous material. A fused siliceous materialfabric, such as one made of fused quartz, and porous carbon have provedespecially effective as filter materials for our purposes. We have founda four-layer composite laminated filter medium comprising respectivelayers, beginning with the side next to the melt to be filtered, of anouter lamina of a fused siliceous material fabric such as fused quartzfabric, alumina-silica paper, carbon or graphite felt and an innerlamina of a fused siliceous material fabric such as fused quartz fabricto be of particular utility. However, omitting the carbon or graphitefelt layer has proved not deleterious. Fused quartz fabric, in additionto being a reasonably good filter medium for the subject environment,additionally provides a sturdy construction which extends the usefullife of the filter. An alumina-silica paper layer affords highresistance to bath flow and aids in the filtering out of unsettledsmaller particulates. Carbon felt, when used, has non-wettingcharacteristics in the molten salt bath such that embrittlement orcracking thereof is minimized, should the bath level drop below thefilter such as to expose it to a gas zone.

Among the advantages of the subject invention is the provision of a meltof high-purity alkali metal chloride for the permitted electrolysis ofaluminum chloride in a continuous economic manner. Other advantages willbecome apparent from the following portions of the speciiication andfrom the appended drawings, which illustrate, in accordance with themandate of the statute, presently preferred embodiments thereofincorporting the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of ourinvention, reference will now be made to the drawing which forms a parthereof.

In the drawing, FIG. l is a schematic flow sheet representation of thealkali metal chloride melt prepurication process of the invention.

FIG. 2 depicts a schematic longitudinal cross section of a filter whichmay be used according to a preferred embodiment of the invention.

In FIG. 1 alkali metal chloride are introduced to furnace 10 heated byelectrical, gas or other conventional means (not shown) via line 12. Thetemperature is raised sufiiciently to form a melt 14 of molten salt ofthe added alkali metal chlorides. Nitrogen or other inert gas, forexample, argon or the like, is introduced via line 16 above molten mass14 in the furnace 10 such that sufficient pressure differential existsto filter the melt through filter 18 and out through conduit 20 toelectrolysis cell 22, wherein aluminum chloride added via line 24 may beelectrolyzed to aluminum 26 (shown settled to the bottom of cell 22),with chlorine, which escapes via line 28, also being formed, the melt incell 22 being designated 30. Aluminum may be introduced to the melt inchamber 10 via line 12 or separate opening (not shown) so as toprecipitate iron and other heavy metal impurities bound beforeprecipitation in the form of chlorides or other halides soluble in themelt. The aluminum also reacts with oxygen or hydroxide as explainedhereinabove to form insoluble alumina, which settles to the bottom ofthe furnace 10 together with the iron and other insoluble solids.Settled particulate and sludge 32 may be removed via outlet 33. l to 2%by weight aluminum chloride may also be introduced via line 12 orseparate opening (not shown), if desired, to react with any hydroxidewhich may be present in the melt and precipitate alumina and formhydrogen chloride gas (when the reacting hydroxide is water). Any solidswhich are not settled to the bottom to form the layer of precipitatesand sludge 32 are prevented by filter 18 from passing on via line 20 tocell 22 and interfering with efficient electrolysis.

The filter 18 may be constructed so as to be substantially impervious tomuch of the solid contaminants and precipitates in the melt. Aluminumcontaining a small amount of alkali metal chloride from the melt may betapped from cell 22 as desired and introduced to settler 34 via line 36.Alkali metal chloride 38 may be skimmed or poured from the surface ofaluminum 39 in settler 34 and then conducted via line 40, and added asdesired, to pre-melt chamber or furnace 10. Substantially pure moltenaluminum may be withdrawn from settler 34 as indicated at line 41 bytapping or the like.

In FIG. 2, the illustrative filter is constructed of four layers wrappedaround the perforated end section 42 of a conduit 44, the respectivelayers being fused quartz fabric 46, carbon felt 48, alumina-silicapaper 50 and again fused quartz fabric S2. According to the preferredembodiment of filter structure useful according to the invention, pipe44 extends into substantially cylindrical perforated filter support 42(shown partly in broken section to illustrate wall perforations) almostto the end thereof. This permits the filter to still permit passage ofmelt therethrough and on into pipe 44 through the open end thereof evenwhen the tip or end of the filter extends only slightly below thesurface of the melt being filtered. The four-layer filter material maybe secured to the perforated support 42 by clamps (not shown), ifdesired.

DESCRIPTION OF THE PREFERRED EMBODIMENT The following example isillustrative of our invention.

Example Two thousand pounds (2000 lbs.) of alkali metal chloride saltcontaining 60% NaCl and 40% LiCl are melted over an 8-hour period in apurification furnace, for

example, as depicted schematically at in FIG. 1. During a second S-hourperiod, 0.5% by weight aluminum (10 lb. Al) and 1-2% by weight AlCl3(20-40 lb. A1013) are added, and the precipitates formed settled to thebottom. A 10 s.c.f.h. (standard cubic feet per hours) inert gas(nitrogen) purge into a vapor zone above the molten salt in the furnaceis used to prevent moisture in ambient air from entering the furnace.'Ihe resulting melt is transferred to an electrolysis cell during athird 8-hour period by pressurizing the purification furnace with inertgas (nitrogen), causing melt to flow through a filter, for example,depicted at 18 of FIG. l and in FIG. 2 and into a transfer pipe leadingto the cell. The filter consists of an 0.05 in.thick layer of fusedquartz fabric, a As in.- thick layer of alumina-silica paper, a 1Ain.thick layer of carbon felt, and a second layer of' 0.05 in.thickfused quartz fabric wrapped around a perforated metal support frame.Flow through the filter is 50-60 1b. melt/ min./ft.2 filter area, with apressure drop of approximately 2 p.s.i. across the filter and transferline. Settled sludge is removed from the bottom of the furnace asrequired.

By using a fritted quartz filter (instead of the 4-layer filter recitedabove), the bath contaminants were decreased from 0.22% oxygen in themelt to 0.025% oxygen, as measured by neutron activation analysis, whenthe stoichiometric quantity of AlCl3 was used. The oxygen was reduced toonly 0.125% when 2.3% excess AlCl3 was used.

While the invention has been described in terms of preferredembodiments, the claims appended hereto are intended to encompass allembodiments which fall within the spirit of the invention.

Having thus described our invention and certain embodiments thereof, weclaim:

1. In a process for electrolytic production of aluminum by electrolysisof aluminum chloride in a cell, said electrolysis being performed in analkali metal chloride electrolyte, the steps comprising forming a meltof at least one alkali metal chloride, the resultant melt containing atleast 75% by weight alkali metal chloride, and feeding at least part ofsaid resultant melt to said cell.

2. The process of claim 1 wherein impurities are removed from the meltprior to feeding same to said cell.

3. The process of claim 2 wherein said impurities are removed byseparation of solids from said resultant melt.

4. The process of claim 3 wherein the separation of solids isaccomplished by settling of the solids and employing resultingsupernatant fluid as the melt with impurities removed therefrom fed tosaid cell.

5. The process of claim 4. wherein the supernatant uid is filtered priorto feeding same to said cell.

6. The process of claim 1 wherein aluminum is removed from said cell andalkali metal chloride melt is separated therefrom and added to said meltprior to feeding same to said cell.

7. The process of claim 1 wherein a precipitating agent is added toprecipitate solids from said resultant melt, and the resultingprecipitated solids, with other substantially insoluble solids in saidmelt, are settled, thereby permitting impurities to be removed from themelt prior to feeding same, with insoluble solids removed therefrom, tosaid cell.

8. The process of claim 7 wherein the precipitating agent comprises upto l to 2% by weight of said melt aluminum chloride.

9. The process of claim 7 wherein the precipitating agent comprisesaluminum or an alloy containing aluminum.

10. The process of claim 7 wherein the precipitating agent comprises upto 1 to 2% by weight of said melt aluminum chloride and in additionaluminum.

11. The process of claim 1 wherein said forming of said melt is effectedin a chamber under an inert atmosphere.

12. The process of claim 1 wherein said resultant melt is passed througha filter prior to feeding same to said cell.

13. The process of claim 12 wherein the filter comprises fibrousmaterial on a perforated support therefor.

14. The process of claim 13 wherein the fibrous material comprisesporous carbon.

15. In a process for electrolytic production of aluminum by electrolysisof aluminum chloride in a cell, said electrolysis being performed in analkali metal chloride electrolyte, the steps comprising forming a meltof at least one alkali metal chloride, the resultant melt containing atleast 75%l by weight alkali metal chloride, passing said resultant meltthrough a filter of siliceous fibrous material on a perforated supporttherefor, and thereafter feeding at least part of said resultant melt tosaid cell.

16. The process of claim 15 wherein the siliceous fibrous materialcomprises respective layers of fused siliceous material fabric,alumina-silica paper and fused siliceous material fabric.

17. The process of claim 15 wherein the siliceous fibrous materialcomprises respective layers of fused silipaper and fused siliceousmaterial fabric.

18. The process of claim 17 wherein the respective first and last fusedsiliceous material fabric layers comprise fused quartz fabric.

19. Apparatus for the preparation of high-purity alkali metal chloridefor use in electrolysis of aluminum chloride to aluminum comprising, incooperative association,

furnace means for forming a melt of alkali metal chloride,

means for maintaining an atmosphere of inert gas in said furnace means,

means for feeding molten alkali metal chloride from said furnace meansto an electrolysis cell,

and means for selectively separating solids from said molten alkalimetal chloride prior to feeding same to said electrolysis cell.

20. The apparatus of claim 10 wherein the means for selectivelyseparating solids comprise a filter.

21. The apparatus of claim 19 wherein the means for selectivelyseparating solids comprise a perforated support having thereon a fibrousfilter material pervious to dissolved alkali metal chloride andimpervious to at least some of the undissolved solid impurities in saidmelt.

22. The apparatus of claim 21 wherein the fibrous filter materialcomprises siliceous material.

23. The apparatus of claim 21 wherein the fibrous filter materialcomprises porous carbon.

24. The apparatus of claim 21 wherein the perforated support issubstantially cylindrical and is joined as a terminal portion to aconduit extending into said furnace means, and the brous filter materialis wrapped around the perforated cylindrical wall of said support.

25. Apparatus for the preparation of high-purity alkali metal chloridefor use in electrolysis of aluminum chloride to aluminum comprising, incooperative association,

furnace means for forming a melt of alkali metal chloride,

means for maintaining an atmosphere of inert gas in said furnace means,

means for feeding molten alkali metal chloride from said furnace meansto an electrolysis cell,

and means for selectively separating solids from said molten alkalimetal chloride prior to feeding same to said electrolysis cell, saidmeans for selectively separating solids comprising a perforated supporthaving thereon a fibrous filter material pervious to` dissolved alkalimetal chloride and impervious to at least some of the undissolvedimpurities in said melt, said fibrous filter material comprising layersof fused siliceous material fabric, alumina-silica paper and fusedsiliceous material fabric.

7 8 26. The apparatus of claim 25 wherein the librous 2,587,328 2/ 1952Johnson 204-67 X filter material has additionally a layer of porouscarbon 3,400,060 9/ 1968 Gallinger 204-276 X between at least one of thelayers of fused siliceous material fabric and the aluminum-silica paper.HOWARD S- WILLIAMS, Pflmafy EXaITIlIleI 5 D. R. ALENTINE, A ReferencesCited V ssrstant Examiner UNITED STATES PATENTS U.S. Cl. X.R.

3,508,908 4/1970 Herwig et al. 204-67 204-245, 246, 276 2,451,49210/1948 Johnson 204-67 UNITED STATES www @emes CERTWECA'EE @ll CHRECHCNPatent No. 3,761, 365 Dated September .25 5 1973 Invencor s Warren E.Haupin and Bernard J. Racunas It is certified that error appears in theabove-identified patent and that said Letters Patent are herebyCorrected as shown below:

Col. 69 Claim 17, After "fused" cancel 'sli" and line 2 v substitute--sliceous material fabric, porous carbon, aluminasilica".

Col. 6, Claim 2O Change "l0" to l9.

line 40 Signed and sealed this 26th day o February 1972.1..

(SEAL) Attest:

EDWARD M.FLETCHER J'R. Attestng Officer', C MARSHALL DANN Commissionerof Patents E FORM PO-105O (I0-69) USCOMM'DC GOS'GIPGQ ILS. GOVERNMENTPRINTING OFFICE: |969 0--866851L/

